U.S. Pat. No. 5,501,098 to Cadet et al., incorporated by reference herein, discloses an acoustic gas composition analysis cell 10 which is comprised of first and second transducer housings 40 for sealing first and second ends respectively of a conduit 20. Each transducer housing 40 includes a transducer 80. One of the ports 70 act as an input port for gas flow and one of the ports 70 act as an outlet port for gas flow. Thus, one of the transducers 80 is located upstream of the gas flow and one located downstream. Because of the doppler shift effect, a measurement of the time of travel of an acoustic signal from one transducer 80 to another, is affected by whether the acoustic signal starts from a downstream transducer and then is transmitted to an upstream transducer or starts from an upstream transducer and then is transmitted to a downstream transducer. I.e., the time of travel measurement is affected by whether the signal is travelling in the direction of the flow of the gas or against the flow of the gas.
U.S. Pat. No. 5,625,140 to Cadet et al., incorporated by reference herein, discloses a gas composition anaylsis cell 10 wherein a sound absorbing sleeve 100 is wrapped around a conduit 20 to reduce noise.
As described in U.S. Pat. No. 5,501,098 to Cadet et. al., incorporated by reference herein, in many manufacturing operations accurate information concerning a gas composition is necessary to control a particular process. (Cadet, 5,501,098, col. 1, lns. 14-30). In line gas monitoring is often used to ensure a consistent delivery of gas. Acoustic analysis can be used to determine the concentration of a component of a gas mixture. The transit time or time of travel of sonic pulses between transducers is measured and used to determine the velocity of sound in the gas. From the velocity, the composition of a binary mixture of gas can be determined. For more details refer to col. 1, ln. 30-col. 3, ln. 4, in Cadet et. al., 5,501,098, which section is incorporated by reference herein.